1. Field of the Invention
The present invention relates to optical fibers and, more particularly, to a method of reducing the loss characteristic of an optical fiber.
2. Description of the Related Art
In general, an optical fiber includes a core and a clad surrounding the core, and the refractive index of the core is higher than the refractive index of the clad. When the optical fiber is fabricated, the optical fiber may contain an OH-radical therein due to a heat source or impurities in the raw material used to produce the optical fiber. The content of the OH-radical may increase under a high-energy radiation. It is difficult for an optical fiber to operate in the range of E-band (1360 nm˜1460 nm) because of the loss characteristic of the fiber caused by the OH-radical. This phenomenon is worse if the optical fiber is exposed to a hydrogen even when the optical fiber has a little amount of the OH-radical. Such an increase in the loss of the optical fiber as a function of time due to the hydrogen is known as “hydrogen aging loss.” The hydrogen aging loss may cause a serious problem when the optical fiber is being used for a long period of time.
In order to solve the above problem, a technique of replacing the OH-radical included in the optical fiber to an OD-radical has been proposed. It involves exposing the optical fiber (or an optical fiber preform) to a deuterium. Since such a replacement reaction is an irreversible reaction, the OH-radical cannot be formed in the optical fiber treated with the deuterium even if the optical fiber is exposed to hydrogen. However, the deuterium remaining in the optical fiber may exert an undesirable influence on the optical fiber when operated for a lengthy time. To address this, a degassing process must be carried out in the optical fiber after a deuterium treatment process has been performed.
U.S. Pat. No. 6,704,485, which is issued to Jean-Florent Campion, discloses a method for reducing the hydrogen sensitivity of the optical fibers at 1380 nm˜1410 nm. Briefly, this method includes the steps of exposing an optical fiber to a gas mixture including a deuterium under a normal temperature (preferably, 20° C.˜40° C.) and a standard pressure (or pressure higher than the standard pressure) so as to allow the optical fiber to make a contact with the deuterium, and degassing the optical fiber in a neutral atmosphere, such as a nitrogen atmosphere or an air atmosphere.
However, a deuterium treatment method and a degassing method disclosed in the above patent has the following problems.
Firstly, when a deuterium treatment time is short, the deuterium is inefficiently diffused into a core. Accordingly, in order to diffuse the deuterium into the core efficiently, the deuterium treatment process must be carried out for a long period of time (1 day to 2 weeks, preferably 3 days to 10 days).
Secondly, since 1 to 3 weeks (preferably 2 weeks) may be required for the degassing process, an excessive process time is required for the degassing process.